In an effort to increase device densities, many years of research have been devoted to reducing critical dimensions (CDs) in integrated circuit devices. Research has resulted in a long felt need to replace traditional gate structures with gates having high-k dielectrics and metal electrodes. High-k dielectrics can provide enhanced capacitance in comparison to an equivalent thickness of silicon dioxide. A metal electrode with suitable work function is desirable to avoid charge carrier depletion proximate the electrode interface with the high-k dielectric. The electrodes for p-channel and n-channel transistors may require different metals to provide suitable work functions.
Suitable metals for gate electrodes can be adversely affected by processing used to form source and drain regions. In particular, annealing to repair source and drain implant damage can shift the work function of electrode metals. This has led to various new manufacturing processes, including replacement gate (gate-last) processes. In a replacement gate process, a gate stack is formed with polysilicon in place of the electrode metal. After source and drain regions are formed, the polysilicon is removed to form trenches which are then filled with the desired electrode metals.